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Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts © 2014 Pearson Education, Inc. What are asteroids like? Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1000 kilometers. 150,000 in catalogs, and probably over a million with diameter >1 kilometer. Small asteroids are more common than large asteroids. All the asteroids in the solar system wouldn't add up to even a small terrestrial planet. © 2014 Pearson Education, Inc. • Asteroids are cratered and not round. © 2014 Pearson Education, Inc. Asteroids with Moons Ida and Dactyl (imaged by Galileo probe) © 2014 Pearson Education, Inc. Asteroid Orbits • Most asteroids orbit in the asteroid belt between Mars and Jupiter. • Trojan asteroids follow Jupiter's orbit. • Orbits of near-Earth asteroids cross Earth's orbit. Why are there relatively few asteroids beyond Jupiter? © 2014 Pearson Education, Inc. But why is there an asteroid belt? Rocky planetesimals between Mars and Jupiter did not accrete into a planet. Jupiter's gravity stirred up asteroid orbits and prevented them from accreting. So what happened to asteroids closer to the Sun than the asteroid belt? © 2014 Pearson Education, Inc. How are meteorites related to asteroids? © 2014 Pearson Education, Inc. Meteor Terminology • Meteoroid: a rock in space • Meteorite: a rock from space that falls through Earth's atmosphere • Meteor: the bright trail left by a meteorite © 2014 Pearson Education, Inc. Meteorite Impacts Small impacts not uncommon. But only one known story of a person getting hit, in 1954. © 2014 Pearson Education, Inc. Meteorite Types 1) Primitive: unchanged in composition since they first formed 4.6 billion years ago 2) Processed: younger; have experienced processes like volcanism or differentiation © 2014 Pearson Education, Inc. Primitive Meteorites © 2014 Pearson Education, Inc. Processed Meteorites © 2014 Pearson Education, Inc. Meteorites from Moon and Mars • A few meteorites arrive from the Moon and Mars. • Composition differs from the asteroid fragments. • A cheap (but slow) way to acquire Moon rocks and Mars rocks © 2014 Pearson Education, Inc. What have we learned? • What are asteroids like? – They are rocky, small, potato-shaped leftovers from the era of planet formation. • Why is there an asteroid belt? – Jupiter’s gravity prevented planetesimals between Jupiter and Mars from forming a planet. © 2014 Pearson Education, Inc. What have we learned? • How are meteorites related to asteroids? – Primitive meteorites are remnants from solar nebula. – Processed meteorites are fragments of larger bodies that underwent differentiation. © 2014 Pearson Education, Inc. 12.2 Comets • Our goals for learning: – What are comets like? – Where do comets come from? © 2014 Pearson Education, Inc. What are comets like? © 2014 Pearson Education, Inc. Comet Facts • Formed beyond the frost line, comets are icy counterparts to asteroids. • Nucleus of comet is a "dirty snowball." • Most comets do not have tails. • Most comets remain perpetually frozen in the outer solar system. • Only comets that enter the inner solar system grow tails. © 2014 Pearson Education, Inc. Anatomy of a Comet A coma is the atmosphere that comes from a comet's heated nucleus. A plasma tail is gas escaping from coma, pushed by the solar wind. A dust tail is pushed by photons. © 2014 Pearson Education, Inc. Growth of Tail © 2014 Pearson Education, Inc. • Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet's orbit. © 2014 Pearson Education, Inc. What does a comet nucleus look like? Rosetta and Comet 67P/Churyumov-Gerasimenko © 2014 Pearson Education, Inc. What does a comet nucleus look like? © 2014 Pearson Education, Inc. Rotating View What does a comet nucleus look like? Rosetta images of comet © 2014 Pearson Education, Inc. How does one land on a comet? © 2014 Pearson Education, Inc. How does one land on a comet? Rosetta’s view of the Philae descent © 2014 Pearson Education, Inc. How does one land on a comet? 3 km away Approaching the landing….. © 2014 Pearson Education, Inc. How does one land on a comet? 40 meters away….. © 2014 Pearson Education, Inc. How does one land on a comet? First image from surface © 2014 Pearson Education, Inc. Rosetta’s final descent © 2014 Pearson Education, Inc. Where do comets come from? © 2014 Pearson Education, Inc. • Only a tiny number of comets enter the inner solar system. Most stay far from the Sun. • Oort cloud: on random orbits extending to about 50,000 AU • Kuiper belt: on orderly orbits from 30–100 AU in disk of solar system © 2014 Pearson Education, Inc. How did they get there? • Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets • Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction © 2014 Pearson Education, Inc. What have we learned? • What are comets like? – Comets are like dirty snowballs. – Most are far from Sun and do not have tails. – Tails grow when comet nears Sun and nucleus heats up. • Where do comets come from? – Comets in plane of solar system come from Kuiper belt. – Comets on random orbits come from Oort cloud. © 2014 Pearson Education, Inc. 12.3 Pluto: Lone Dog No More • Our goals for learning: – How big can a comet be? – What are the large objects of the Kuiper belt like? © 2014 Pearson Education, Inc. How big can a comet be? © 2014 Pearson Education, Inc. Pluto's Orbit Pluto will never hit Neptune, even though their orbits cross: • Because of the tilt of Pluto’s orbit, and • Because their 3:2 orbital resonance (Neptune orbits three times during the time Pluto orbits twice) keeps the two safely away from one another. © 2014 Pearson Education, Inc. What is Pluto like? Its moon Charon is nearly as large as Pluto itself (probably made by a major impact). Pluto is very cold (40 K). Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto's orbit takes it farther from the Sun. © 2014 Pearson Education, Inc. Pluto Images from Hubble Space Telescope © 2014 Pearson Education, Inc. Pluto New Horizons (2015) Pluto and Charon © 2014 Pearson Education, Inc. Pluto New Horizons Icy mountains and flat ice plains © 2014 Pearson Education, Inc. Pluto New Horizons Glacial Flows © 2014 Pearson Education, Inc. Is Pluto a Planet? • Much smaller than the terrestrial or jovian planets • Not a gas giant like other outer planets • Has an icy composition like a comet • Has a very elliptical, inclined orbit • Has more in common with comets than with the eight major planets © 2014 Pearson Education, Inc. Other Icy Bodies There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. The largest of these, Eris, was discovered in summer 2005, and is even larger than Pluto. © 2014 Pearson Education, Inc. Kuiper Belt Objects • These large, icy objects have orbits similar to the smaller objects in the Kuiper belt that become short period comets. • So are they very large comets or very small planets? © 2014 Pearson Education, Inc. Pluto and Eris • Pluto's size was overestimated after its discovery in 1930, and nothing of similar size was discovered for several decades. • Now other large objects have been discovered in Kuiper belt, including Eris. • The International Astronomical Union (IAU) now classifies Pluto and Eris as dwarf planets. • Dwarf planets have not cleared most other objects from their orbital paths. © 2014 Pearson Education, Inc. What have we learned? • How big can a comet be? – The Kuiper belt from which comets come contains objects as large as Pluto. • What are the large objects of the Kuiper belt like? – Large objects in the Kuiper belt have orbits and icy compositions like those of comets. © 2014 Pearson Education, Inc.